Nitrocellulose based propellants are used in the initiation of ammunition systems such as rockets and projectiles. In firearms, propellants fill the interior of an ammunition cartridge chamber of a gun or cannon, leading to the expulsion of a bullet or shell.
Solid propellants are categorized according to the number of active components in the propellant. For instance, single-based propellants primarily contain low-order explosive powder such as nitrocellulose. Double-base propellant, consists of nitrocellulose and nitroglycerine, plus additives in small quantity. The molecules are unstable, and when ignited, they break apart and rearrange themselves, liberating large quantities of heat. These propellants lend themselves well to smaller rocket motors. They are often processed and formed by extrusion methods, although casting has also been employed.
Numerous additives are used in solid propellants to improve their burn rate, stability, sensitivity, and mechanical properties. Additives such as plasticizers may be used to process fluid materials in the propellant mix and improve the mechanical properties of the propellant compositions. Nitroglycerin, a liquid explosive, may also function as a plasticizer. While nitroglycerin has the dual properties of being an energetic and a plasticizer, it has the tendency to migrate throughout a propellant grain leading to performance and safety problems. For instance, long term storage of nitroglycerin-based propellants has caused explosions and fires due to migration of the energetic material through the propellant matrix.
Diffusion and migration of propellant constituents can also arise with co-extruded propellants. A co-extrusion propellant is a dual system with an inner and outer core having its own constituents independent of one another to control the propellant's burn rate. The inner core is known for its high energy density and burns much hotter than the outer core. The outer core may be formulated with nitrocellulose which acts to minimize the extremely hot burn of the inner core. The purpose is to have the plasticizers that burn hot contained in the inner core of the propellant, leaving the outer core as a single-based nitrocellulose. Problems arise when plasticizers such as nitroglycerin diffuse into the outer portion of the propellant grain thus, defeating the intended purpose of the co-extruded grain.
One solution is to coat the propellants, however, these coatings do not effectively contain the plasticizers. This is due to the presence of cracks and voids that are naturally produced during a coating process. Furthermore, propellant coatings affect the performance of the propellant and poses incompatibility issues over time. For instance, some coatings are useful only in low temperature and humidity environments. When environmental conditions fluctuate, the components may continue to migrate.
Accordingly, a need exists to prevent migration or diffusion of components such as nitroglycerin in propellant compositions without compromising the desired energetic properties of the propellant.